feat: WebDAV file access and category color sync (v0.2.0)

Major Changes:
- Switch from Nextcloud Notes API to direct WebDAV file access
- Notes stored as .txt files with filename-based IDs for reliability
- Implement safer sync strategy without clearNotes() to prevent data loss
- Add ETag-based conflict detection for concurrent edits
- Add category color sync to .category-colors.json on server
- Show neutral gray badges for categories without assigned colors

Technical Improvements:
- Replace numeric IDs with filename-based string IDs
- Update Note type to support both number and string IDs
- Implement WebDAV methods: fetchNotesWebDAV, createNoteWebDAV, updateNoteWebDAV, deleteNoteWebDAV
- Add CategoryColorsSync service for server synchronization
- Remove hash-based color fallback (only show colors when explicitly set)

Bug Fixes:
- Fix category badge rendering to show all categories
- Prevent note loss during sync operations
- Improve offline-first functionality with better merge strategy

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "Nextcloud Notes",
-  "version": "0.1.5",
+  "version": "0.2.0",
   "identifier": "com.davidrelich.nextcloud-notes",
   "build": {
     "beforeDevCommand": "npm run dev",